It is known to provide speed-change transmissions with speed-selection clutches which operate in an oil bath or housing and which are provided with passages enabling lubricating oil to be supplied to the clutch at least during the actuation thereof. For example, German Federal Patent Publication (open application or Offenlegungsschrift) DT-OS No. 2,215,922 (see U.S. Pat. No. 3,834,503) provides a control member or system for the supply of lubricating oil to a speed-selection clutch of the latter type which is controlled in response to the input speed.
In this system and in accordance with one of the embodiments of the arrangements described therein, it has already been proposed to provide a separate oil flow which is independent of any fluid displacement during clutch application, for cooling and lubricating the stacks of clutch lamellae. The distribution and feed of the oil is effected through a plurality of valve members which are provided with locking disks and pins forming slide valves.
In this system, however, it is not possible to ensure effective distribution as well as a quantity of lubricant in accordance with heat developed by the clutch.
Furthermore, earlier systems for metering the lubricating and cooling oil to such clutches have a large number of components especially provided for the metering purpose and requiring small tolerances during fabrication if proper functioning is to be achieved. The systems are not conductive to mass production because they require high fabricating precisions and are not readily accessible for maintenance and repair.
More generally, the earlier systems are neither economically nor functionally satisfactory and cannot be considered reliable over long periods of use. They do not minimize effectively the lubricating oil flow during inoperative conditions of the clutch so that relatively large amounts of oil must be displaced even though lubrication is not as necessary. In fact, most earlier systems maintain a practically constant flow of oil independent of the lubricating needs of the clutch.
Mention can also be made of the fact that earlier clutches with oil flow control have the significant disadvantage that the lubricating or cooling of the clutch during the nonactuated state and during the start of clutch actuation is effected with relatively low pressure. Separate means is generally provided for actuating the clutch and such means is usually totally independent of the oil flow. The actuation, for example, may be effected by means of an annular piston which is axially shifted by the supply of a hydraulic fluid separate from the oil flow and which acts against a restoring spring urging the annular piston into its disengaged position. The use of such springs, annular pistons and like members in these systems has created problems with respect to the spring itself, because of material fatigue, and the guides for the cylinders which may bind in a hot state of the clutch, especially if the lubricant flow is not sufficient. In addition, many constructions make it difficult to provide a sufficiently large or stiff spring for uniformly pressing the piston back into its starting position because of the desire for overall compact construction of the device.